Canon AT 1 - Manual (Page 51)

Viewing and Focusing
Focusing is performed in the small round
area in the center of the viewfinder. The
smaller central circle is a split-image focusing
screen and around it is the microprism ring.
The split-image rangefinder ascertains that the
image is "in focus" when the image divided
horizontally in half merges and becomes one
complete image.
The microprism rangefinder presents a
clear and steady image when in focus. The
microprism conveys a broken, shimmering
image when not accurately in focus. It is also
possible to focus with the matte screen
outside the smaller central area. You can
focus with any of these focusing aids as you
like, depending on the subject and your pre­
ference.
Accessories such as an eyecup, dioptric
adjustment lenses, angle finders, and magnifier
can be attached to the viewfinder eyepiece.